1. Field of the Invention
The present invention relates to circuits having a capacitor for storing a reference voltage, and more particularly, to such circuits for which the reference voltage varies.
2. Discussion of the Background of the Invention
Many circuits such as analog to digital converter circuits require a reference voltage for comparison and other purposes. This reference voltage is usually supplied by an external source and is typically stored on a reference capacitor. For a number of applications including data acquisition circuits, the reference voltage from the reference source does not always remain constant. Instead, the reference voltage may change to accommodate a change in the polarity or magnitude of the data to be processed. However, the internal circuitry of the data acquisition circuitry typically cannot react instantly to changes in the voltage of the reference source due to the capacitance of the reference capacitor. Instead, it.usually takes a finite period of time to charge (or discharge) the capacitor to the new reference voltage. This charging period can often be as long as several seconds where the output impedance of the reference source is large. As a consequence, the data acquisition circuitry may output improper readings while the voltage across the reference capacitor is settling to the new voltage supplied by the reference source.
Sample and hold amplifiers have been designed to increase the charging rate of storage capacitors. However, these amplifiers have generally been intended for relatively low accuracy applications.